Ligamentous stability of the knee will be quantified In-Vivo and In-Vitro by direct measurement of force vs displacement responses for anterior-posterior tibial drawer, moment vs rotation responses for varus-valgus angulation of the tibia, and torque vs rotation responses for internal and external rotation of the tibia. The stiffness and laxity data collected will quantify the contributions of knee ligaments to overall stability of the joint, and help to improve the accuracy and interpretation of the clinical knee laxity exam. Existing test fixtures will be modified to study the complex responses of cadaveric knees to combined loading states on an MTS materials test machine, including the application of tibial-femoral contact force (joint load). The effects of knee ligament section and total knee replacement will be examined with this advanced methodology. The UCLA clinical knee testing apparatus will be modified to include measurement of anteromedial and anterolateral rotatory instabilities. Patients with injuries to their knees will be tested before and after their surgical reconstructive procedures to permit an objective assessment of their operative results. Patients who have received total knee replacements will undergo selected testing to evaluate the stability of their implants in-situ. A portable "field" testing apparatus will be designed and prototypes constructed for use in the UCLA clinics and Sports Medicine Center. These units will expand our data collection capabilities and allow screening studies of varsity athletes prior to the competitive season, in order to study correlations of knee stiffness and laxity with the incidence and severity of knee injury.